Antigen mediated histamine release from cultures of RBL-2H3 cells is associated with increase in cytosol Ca++ levels (Ca signal) and substantial hydrolysis of membrane inositol phospholipids. Several clones of the RBL-2H3 cell line showed varied responses to antigen that ranged in extent from undetectable (BUDR 1A3, 2B1 and 1B3) to about 80% of those in 2H3 cells (TG 2B6). The initial rate of response in the partially responsive clones was similar to that of 2H3 cells but the maximal responses were blunted. In most of these clones, as in the 2H3 cells, the Ca signal (as measured by quin 2 fluorescence) and hydrolysis of the phospholipids were correlated. However, TG 1B3, which showed very little Ca signal, still showed modest phospholipid hydrolysis and histamine release. Phospholipase C activity towards all inositol phospholipids was present in extracts and membranes of all the clones tested. Moreover, activity in the nonresponsive clones was 3 to 5 times higher than that in 2H3 cells. Studies with phorbol ester and Ca2+ ionophore also indicated the presence of protein kinase C activity in the 1A3 and 1B3 clones. These data point to no obvious defect in the genetic expression of enzymes involved in the inositol phospholipid cascade system in the nonresponsive clones. Our preliminary indications are that these clones have defective coupling of IgE receptors to phospholipase C through G-protein(s).